The IMF of the Orion Nebula Cluster Across the H-burning Limit

Nicola Da RioHST Orion Treasury Science Meeting

STScI, Baltimore, September 12th 13th 2011

Motivation• In order to derive the properties of PMS stars in

Orion we need individual spectral types.• We cannot derive spectral types from broad-band

photometry U B V R I z J H K

• Most of the available spectral types are for stellar objects (Hillenbrand 1997, Da Rio+2009)

• In the BD mass range, spectral types are available for a small sample of sources (Slesnick+2004, Riddick+ 2007 ..)

accretion Teff – AV degeneracy

disk excess

Observational idea• Deriving spectral types of cool stars and brown

dwarfs from medium band photometry• Deep photometry with

WFI@MPG/ESO• (35’x35’ FOV) in 4 bands.• Two bands (753 and 770) are

anti-correlated with the spectral type for T<4000K

• I-band is needed to get the extinction star by star

MB 571/25ESO#863

MB 753/18ESO#848

MB 770/19ESO#849 BB Ic/Iwp

ESO#845

Band Total exposure time

753 nm 9360 s

770 nm 14000 s

I-band 3360 s

571 nm 11500 s

Color-color diagram

synthetic isochrone

Color-color diagram

synthetic isochrone

cooler hotter ר

M0

= 37

50K

Color-color diagram

M0

= 37

50K

synthetic isochrone:inaccurate for verylow temperatures

Highly reddened early types

(mostly BKGcontaminants)

VLMS and BDs(right to left)

Possibly members

Low AV early type

members:NOT PRESENT BECAUSE OF

SATURATATION

Background red giants

Photometric depthHST/ACS Z vs (I-Z) CMD

Black: all the ACS sources with I and Z

Green: sources included in our previous work (Da Rio+ 2009, 2010), having WFI V,I and available spectral type

Red: sources for which now I have photometry in all MB753, MB770 and I-band, and are placed in the 2-color diagram shown earlier.

Derivation of Teff e and AV

Reddening independent [770 index] Extinction related [AV index]

RV=3.1 from the Cardelli (1989) law

Derivation of Teff e and AV

• Calibration of the [770 index]-Teff relation based on available spectral types

• The index correlates with temperature for Teff < 3200K

• Candidate accretors are more scattered and systematically shifted.

• What is the effect of accretion excess on the [770 index]-Teff ?

Effect of accretion excess on the [770 index]Displacement of the index as a function of Laccr/Lstar and Teff computed by synthetic photometry

Bias in the determined Teff if the excess is neglected.

The overall effect of accretion veiling is usually neglibile

Calibration of the [AV index]De-reddened sources (based on available AV from the literature and from de-reddening ISPI photometry) allow to define the AV=0 locus

The AV=0 locus nicely confines the observed photometry at positive extinctions

•Teff and AV is derived for 1280 sources• 544 of them previously assigned a spectral type• 527 extra sources have Teff known and > 3200 K:

•1807 sources in Orion have now a Teff estimate

The H-R diagram

Distance: 414pc (Menten et al, 2007)Bolometric correction: from synthetic photometry with empirical calibration

Background contamination• The number and relative fraction of background sources

increases with fainter luminosities (see Robberto+ 2010).

• We detect a population of under-luminous sources well separated from the young population.

• Given the spectral type >M3 these are background red giants

• ONC members seen in scattered light (e.g. Guarcello+ 2010), if present, are a neglibile fraction of these sources.

The H-R diagramWe detect BDs down to 0.02Msun

Masses (and ages) are assigned by interpolation of tracks and isochrones

We use Baraffe et al (1998) and D’Antona & Mazzitelli (1998) models

Completeness in the HRD Photometric completeness (based on artificial star tests preserving spatial distribution of the ONC

Monte Carlo method to convert completeness from observed quantities to the HRD, accounting for differential extinction

Completion accounting for spectral types from previous works

The IMF

•We measure a (completeness corrected) IMF which shows a strong deficiency of substellar objects. •This trend was observed in previous studies (Hillenbrand & Carpenter 2000, Muench 2002, Slesnick 2004), but not as prominent as in this work.•A possible explanation for this difference is our better characterization of field contamination.

Chabrier log-normalKroupa

Conclusions1. Using a time-efficient pratical innovative medium-band photometric

tecnique, we nearly doubled the sample of ONC sources with available spectral types and derived individual extinctions• This sample is complete down to the H-burning limit and extends down

the BD mass range• Optical excess due to mass accretion does not affects this measurement

2. We detect a large population of background contaminants, which dominates over the ONC members in the BD mass range. In the HRD it is possible to exclude this contamination with high accuracy.

3. We derive the IMF for the ONC, which shows steep decline below the H-burning limit, suggesting a deficiency of substellar objects

The paper has been sumbitted to ApJ